Bumper beam attachment structures including a bumper beam attached to right and left side frames of the vehicle body via shock-absorbing extension members are known. One example of such known bumper beam attachment structures is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2001-58550.
The disclosed bumper beam attachment structure, as shown here in FIG. 17, includes a bumper beam 315 attached via a shock-absorbing extension member 314 to each of the right and left rear side frames (left one 311 being shown) of a vehicle body. For attachment, the rear side frame has a reinforcement panel 312 connected by welding to a rear end thereof, and a retaining member 313 connected by welding to the reinforcement panel 312. The extension member 314 is connected to the retaining member 313 by a screw fastener (not designated) so that the bumper beam 315 is attached to the left rear side frame 311 of the vehicle body. The extension member 314 is formed from a sheet metal and has a generally box-shaped configuration with one side 316 open and attached by welding to the bumper beam 315.
With the bumper beam attachment structure 310 thus arranged, when the bumper beam 315 is subjected to an impact force applied from the back of the vehicle body, opposite sidewalls 317 of the box-shaped extension member 314 undergo buckling to thereby absorb the impact force before the impact force is transmitted to the rear side frame 311. In this instance, however, since the impact force acts in a direction substantially parallel to respective general planes of the sidewalls 317, the sidewalls 317 show a great resistance against yielding with the result that a desired shock-absorbing effect cannot be attained. Furthermore, the buckling of the sidewalls 317 occurs suddenly but not gradually, making it difficult to achieve a smooth shock-absorbing operation.
Some sophisticated bumper beams have an additional sock shock-absorbing member attached to a front side thereof for the purpose of absorbing a relatively small impact force by the bumper beam itself when the vehicle body encounters a minor collision, for example. One example of such bumper beams is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2004-155313.
The disclosed bumper beam, as shown here in FIG. 18, includes a bumper beam body 321, a shock-absorbing member 322 attached to a front side of the bumper beam body 321, and a bumper face 323 covering the front side of the bumper beam body 321 including the shock-absorbing member 322. The shock-absorbing member 322 is so constructed as to yield or deform when subjected to a relatively small impact force. To this end, the shock-absorbing member 322 has a hollow structure blow-molded from a synthetic resin material such as polypropylene or polyethylene. The hollow shock-absorbing member 322 includes a base portion 324 attached to the bumper beam body 321, a dome-like front portion 325 projecting forward from the base portion 324, and two reinforcement ribs 326, 326 extending from the base portion 324 to the backside of the front portion 325.
With this arrangement, when the bumper beam 320 is subjected to a relatively small impact force at a light or minor collision with an obstacle 329, the reinforcement ribs 326 of the shock-absorbing member 322 serve to hinder smooth yielding or deformation of the dome-like front portion 325. Depending on a direction of collision relative to the obstacle 322, the shock-absorbing member may be subjected to a rotational moment or torque tending to twist the bumper beam body 321 about a joint between itself and the vehicle body. This may result in a deformation or damage on the bumper beam body 321 and an extension member disposed at the joint between the bumper beam body 321 and the vehicle body.